This invention relates to a method for obtaining in large volume a gas stream that is from 90% to 99% and higher by volume in one component of a gas mixture. This invention especially relates to an adsorption process for providing an enriched gas stream by means of a pressure swing adsorption (PSA) system using molecular sieves. More particularly, this invention relates to a method for providing an inexpensive and high volume source of gases such as nitrogen, hydrogen or methane, requiring less energy to operate than either cryogenic or other pressure swing adsorption systems, and yet supplying gases of comparable quality.
The point at which adsorption has ceased and the gas exiting the adsorbent was essentially the same in composition as the gas that entered the adsorbent is known as the breakthrough point. At some time prior to this breakthrough point, determined by either product gas purity or a timed cycle, the adsorbent must be regenerated.
The term gas mixture, as used herein, refers to the mixture of gases to be separated such as, air and other gas mixtures primarily comprised of two or more components of different molecular size. The terms enriched gas or product gas refer to a gas comprised primarily of that component of the gas mixture relatively unadsorbed after passage of the gas mixture through an adsorbent. For example, enriched gas may be comprised of from 90% to 99% and higher of the unadsorbed component of a gas mixture. The term vent gas as used herein, refers to that gas released countercurrently from the adsorbent column after the product fraction has been removed.
As described herein, gas normally flows in a cocurrent direction, i.e., into the column inlet and out of the column outlet. Gas flowing in the reverse direction, i.e., out of the column inlet is said to flow in a countercurrent direction.
A gas mixture may be fractionated, or separated, using pressure swing adsorption by passing the mixture at an elevated pressure through an adsorbent which is selective in its capacity to adsorb one or more of the components of the mixture. This selectivity is governed by the pore size distribution in the adsorbent and the total pore volume. Thus, gas molecules with a kinetic diameter less than or equal to the pore size are retained, or adsorbed, on the adsorbent while gas molecules of larger diameters pass through the adsorbent. The adsorbent, in effect, sieves the gas according to its molecular size.
Pressure swing adsorption processes usually include at least two columns of adsorbent so that while one column is being regenerated, the other is in the adsorption phase producing enriched product gas. Thus, by cycling between the columns product gas is delivered constantly. A pressure swing may be defined as the change in pressure associated with an adsorption cycle. For example, many PSA systems have a pressure swing from some positive pressure (above atmospheric or 0 psig) to a lower pressure, for example, atmospheric pressure (0 psig).
The use of carbon molecular sieves for the production of enriched nitrogen from air is a known process. These sieves posses a pore structure with a diameter comparable to the kinetic diameter of oxygen. Generally, these sieves are made from coconut, wood, or various forms of coal. See for example; Munzner et al., U.S. Pat. Nos. 3,801,513 and 3,962,129 and Juntgen et al., U.S. Pat. No. 4,124,529.
Also well known is the use of a pressure swing adsorption system for the separation of gas mixtures. See for example, McCombs et al., U.S. Pat. No. 4,194,890 wherein a pressure swing adsorption system employing a product gas purge and an inlet equalization step is described. The primary distinction between McCombs et al. and the instant process is that McCombs et al. requires a costly segregated adsorber (column) in addition to the two main adsorbers. This necessitates several additional partial pressure equalization steps during the adsorption cycle, including two inlet equalization steps per half cycle, compared to only one such step per half cycle for the instant process.
Other PSA systems for the separation of gas mixtures are known in the art. These include; Skarstrom, U.S. Pat. No. 2,944,627; Meyer, U.S. Pat. No. 3,891,411; Walter, U.S. Pat. No. 3,977,845 and Lee et al., U.S. Pat. No. 3,788,036.
Typical problems in the present pressure swing adsorption and molecular sieve technology include; low yield of product gas, large amounts of molecular sieve required, energy inefficient regeneration methods, use of costly vacuum systems and air receivers.